Lithium ion batteries (LIBs) are the dominant secondary (rechargeable) energy storage source for portable and electric vehicle applications. Sodium (Na) ion batteries (referred to as NIBs or SIBs) have attracted scientific attention as alternatives to LIBs, since sodium is more readily available than lithium and has a potential for significant associated cost reduction. Moreover, NIBs are considered as the key technology for meeting large-scale energy storage needs, mainly due to much more geographically democratic availability of Na and lower cost as compared to Li. NIBs also offer an increased resistance to metal plating-induced shorts. The standard electrode potential is determined by the redox couple and by the ion solvation interactions, with the difference between Li and Na standard potentials in carbonate solvents being in the range 0.2-0.25 V.
In LIBs, there is a wide diversification of cathode materials from LiCoO2 to other Li-transition metal oxides such as spinel LiMn2O4 and Li-transition metal phosphates, especially LiFePO4, as well as improved and safer electrolyte chemistries. However, for anodes, graphite is still the primary material in commercial Li-ion batteries having been this way since their introduction.
Likewise, several classes of cathode materials have been proposed for NIBs, including Na0.44MnO2, Na0.85Li0.17Ni0.21Mn0.64O2, Na0.7CoO2, Na3V2(PO4)2F3, Na2FePO4F, LiFeSO4F, and Na4−αM2+α/2(P2O7)2 (⅔≤α≤⅞), M=Fe, Fe0.5Mn0.5, Mn), olivines, and NASICONs. NIB anodes, on the other hand, present more of a challenge since commercial graphite has very low Na storage capacity. Charge storage capacities and cycling stabilities approaching LIB graphite have been demonstrated for various amorphous or partially graphitic carbons. Anodes based on titanium oxide, such as Na2Ti3O7 and anatase TiO2, have also been successfully employed. These are highly desirable from a cost and environmental friendliness perspective, while offering capacities of ˜150 mAhg−1 (milliamp-hour per gram) and good cycling stability. These materials along with the carbons represent perhaps the most economical anode option for large-scale stationary applications.
Other group 14 elements, besides carbon, have potentially higher storage capacities for lithium and sodium. According to the equilibrium phase diagram, tin (Sn) can store 3.75 Na/host-atom (Na15Sn4), with a resulting maximum charge storage capacity of 847 mAhg−1. The experimentally measured capacity of Sn anodes generally approach this value early in testing, but degrades during cycling. For instance, Yamamoto et al. reported a NIB negative electrode based on a Sn thin film with a discharge (charge) capacity of 790 (729) mAhg−1 in the first cycle. However, this electrode showed a rapid capacity decay after 15 cycles. Ellis et al. also observed an initial discharge capacity of ˜850 mAhg−1 for a sputtered Sn electrode and a rapid cycling-induced capacity degradation to near zero. Sn-based alloy composites have been reported to exhibit improved cycling stability, such as (Sn0.5Co0.5)1−xCx alloy, (Cu6Sn5)1−xCx, SnSb/C nanocomposite, Cu6Sn5, Sn0.9Cu0.1 alloy, and Sn—SnS—C nanocomposite.
Antimony (Sb) has also been recently examined for its potential as a NIB anode. The maximum stoichiometry of Na—Sb alloys is Na3Sb, giving Sb a theoretical capacity of 660 mAhg−1. Sb alloy and intermetallic electrodes have been examined, including Cu2Sb with a capacity of 280 mAhg−1, AlSb with a capacity of 490 mAhg−1, Mo3Sb7 with a capacity of 330 mAhg−1, and Sb-MWCNT nanocomposites with a capacity of ˜500 mAhg−1. Germanium (Ge) in thin film form or as porous nanocolumnar structures has been demonstrated to work as a NIB anode as well. Experimental capacities in the range 1:1 NaGe (369 mAhg−1) have been reported.
While binary and several ternary (containing C) Sn- and Sb-based alloys have been examined as potential NIB anodes, little is known regarding Ge-containing systems.
It is desired to develop new anode materials for lithium ion and sodium ion batteries to achieve higher capacity.